1. Field of the Invention
The present invention relates to a method for processing seismic data and to a method for creating a gain curve for seismic data. In another aspect, the present invention relates to a method for processing seismic data to compensate for seismic transmission loss caused by the subsurface, and to a method for creating a gain curve of the seismic transmission loss. In yet another aspect, the present invention relates to a method for processing seismic data to compensate for seismic transmission loss caused by the subsurface by utilizing a fitted decay curve, and to a method for creating such a fitted decay curve. In still another aspect, the present invention relates to a method for processing seismic data to compensate for seismic transmission loss caused by the subsurface by utilizing a fitted decay curve, while reasonably maintaining the temporal and spatial relationships of the seismic data, and to a method for creating such a gain curve.
2. Description of the Related Art
Seismic exploration generally involves generating seismic pulses at the surface of the earth by means of one or more seismic sources. The seismic pulses travel downwardly into the earth with a fractional amount being reflected and/or refracted due to differences in elastic properties at the interface of various subterranean formations.
Detectors, such as seismometers, geophones or hydrophones, produce analog electrical seismic signals or seismic trace signals in response to detected seismic wave reflections and/or refractions. The analog electric seismic signals or seismic trace signals from the detectors can then be recorded. Alternatively, the analog seismic signals or seismic trace signals from the detectors can be sampled and digitized prior to being recorded. The seismic data recorded in either manner are subsequently processed and analyzed to determine the nature and structure of the subterranean formations.
From the recorded data, a seismic section is generated. A seismic section is a seismic image depicting the subsurface layering of a section of earth along a seismic line of profile. The seismic section is an important tool which the geologist studies to determine the nature of the earth's subsurface formations. However, before an array of seismic samples can be converted into a seismic section which can be interpreted by the geologist, the seismograms must be processed to reduce the degradation due to noise.
Seismic interpretation generally involves the study of the behavior of arrival times, amplitudes, velocities, frequencies, and character of the reflections from target horizons. Any changing or anomalous behavior is of particular interest.
It is well known and documented that the amplitude of a seismic signal decays as it propagates through the earth. Further, this amplitude decay will be frequency dependent in that the higher frequency components tend to suffer greater amplitude attenuation particularly at later arrival times. Generally several factors are viewed as contributing to the amplitude attenuation such as geometrical spreading, reflection absorption, scattering and various other transmission loss mechanisms.
This unwanted amplitude attenuation effect can distort or even ruin the seismic data by obscuring or masking seismic events related to the reflections and/or refractions from the subterranean formations.
Prior art methods for processing seismic data to compensate for such amplitude attenuation effects exist.
Amplitude decay typically has been compensated for by application of an inverse gain correction in the form of a programmed gain curve, automatic gain control circuit, exponential gain or other similar method. Such methods correct for amplitude decay as a function of time over the time span of the seismic trace by systematically amplifying the later arriving signals. Additionally, a family of gain corrections keyed to the source-to-receiver distance are sometimes applied, resulting in a scaling as a function of time and position.
Methods such as automatic gain control or exponential gain utilize a scalar that scales a statistical measure of amplitude over a window to some constant value. The length of the scaling window will vary from a few hundred milliseconds to total trace length. The resulting trace had little amplitude variation from zero time to the end of the data. These methods distort both temporal and spatial amplitude relationship along the seismic line because every trace window is scaled to the same constant. An alternative way of computing the transmission loss in the data is to multiply each trace by a single exponential. This method preserves lateral amplitude relationships but may not compensate for transmission loss because it assumes a constant rate of decay over the entire section.
Distortion of the temporal and/or spatial amplitude relationships makes itself very evident in those cases in which well data is available as a confirmation or check of the seismic processing.
U.S. Pat. No. 3,671,930, issued Jun. 20, 1972 to Mateker, Jr. discloses a method of seismographic exploration by determining the attenuation of reflected seismic signals in a geologic section at a first station and a reference station. The reference station recording is converted to a reflectivity function, and the first station recording is divided by the reflectivity function to obtain a desired amplitude function. The logarithm of the desired amplitude function is then determined at a plurality of discrete travel times; and the value of each logarithm is divided by the corresponding value of the discrete travel time to obtain an attenuation coefficient.
U.S. Pat. No. 4,312,050, issued Jan. 19, 1982 to Lucas discloses multidimensional amplitude scaling of seismic data in which each seismic trace associated with a common source-to-receiver distance is first separated into a plurality of frequency band-limited component traces. A time-variant amplitude scale factor is then generated for each component trace. Next, these scale factors are then applied to the component trace thus compensating for amplitude decay of the component trace. Recombination of the component traces creates a multidimensional amplitude scaled seismic trace.
U.S. Pat. No. 4,884,247, issued Nov. 28, 1989 to Hadidi et al., discloses a method of processing geophysical data to compensate for earth filter attenuation.
U.S. Pat. No. 4,964,102, issued Oct. 16, 1990 to Kelly et al., discloses a method for enhancing and evaluating seismic data in which seismic signals are processed to suppress random noise and coherent noise and to enhance primary reflection events while preserving geologically-induced amplitude variations as a function of range for the primary reflection events.
U.S. Pat. No. 5,189,644, issued Feb. 23, 1993 to Wood, discloses the removal of amplitude aliasing effect from seismic data by utilizing a corrective factor, the ratio of amplitude of the seismic data to its trace envelope, prior to transformation from the X-T to the F-K domain. The data is then transformed to the F-K domain where it is subject to filtering. The data is then inverse transformed to the original X-T domain. An inverse corrective factor is then applied, restoring the amplitude of the reflection signal component of the data.
U.S. Pat. No. 5,197,038, issued Mar. 23, 1993 to Chang et al., discloses a seismic processing method which utilizes a sonic tool for quantitatively determining parameters of a velocity profile of an altered zone in a seismic formation traversed by a borehole. In the method, synthetic amplitude information for the formation is generated by a computer model, by providing the computer with a proposed velocity profile for the formation, including values for parameters such as the radius of the altered zone and the acoustic velocities in the altered zone. Next, the synthetic amplitude information is compared with amplitude information of a compressional headwave as measured by the sonic tool located in the borehole.
While prior art methods for compensating for effects of amplitude attenuation accomplish the task to a degree, they suffer from one or more limitations. For example, utilizing scalar corrections will distort temporal and/or spatial amplitude relationships of the seismic data.
Thus, while prior art techniques exist for compensating for effects of amplitude attenuation each of these suffers from one or more limitations and improvement in the techniques is desired.
Therefore, a need exists in the art for an improved method for compensating for effects of amplitude attenuation without the limitations of the prior art.